Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 12.5	$ 4.1	$ 45.9	$ 24.9
Other comprehensive income (loss):
Foreign currency translation adjustment	0.2	(0.3)	1.5	(1.3)
Income tax (expense) benefit on foreign currency translation adjustment		0.1	(0.2)	0.5
Comprehensive income	12.7	3.9	47.2	24.1
Comprehensive (income) loss attributable to noncontrolling interest	(0.2)	(0.2)	(0.6)	(0.3)
Comprehensive income attributable to Global Brass and Copper Holdings, Inc.	$ 12.5	$ 3.7	$ 46.6	$ 23.8